All those in the study had older siblings. Some of the elder children had autism and some did not.

Since autism can run in families, researchers decided to look at whether vaccines could make autism even more likely in children who had siblings with autism.

Researchers led by Dr Anjali Jain from Falls Church, Virginia found that vaccines had no effect on autism risk, whether or not a sibling in the family was diagnosed.

"Consistent with studies in other populations, we observed no association between MMR vaccination and increased ASD risk among privately insured children," write the study's authors.

"We also found no evidence that receipt of either one or two doses of MMR vaccination was associated with an increased risk of ASD among children who had older siblings with ASD," they report.

Autism is on the rise, but its causes remain poorly understood.

Fears about vaccines and autism began to spread after the publication in 1998 of an article by Andrew Wakefield that purported to find a link between the MMR vaccine and autism in 12 children.

It was later found to be fraudulent and was retracted by the journal that published it. Britain has also stripped the author of his medical licence.

But concerns over vaccine safety, particularly in the internet age, have proven difficult to quell.

"Although a substantial body of research over the last 15 years has found no link between the MMR vaccine and ASD, parents and others continue to associate the vaccine with ASD," say the researchers.

"Surveys of parents who have children with ASD suggest that many believe the MMR vaccine was a contributing cause."

Children who have an older sibling with autism are less likely to get vaccinated, the study found.

The MMR vaccination rate for the children with unaffected siblings was 92 per cent by age five.

In contrast, the MMR vaccination rates for children with older siblings with ASD was 86 per cent by age five.

In an accompanying editorial in JAMA, Dr Bryan King, a doctor at the University of Washington and Seattle Children's Hospital, says the data is clear.

"The only conclusion that can be drawn from the study is that there is no signal to suggest a relationship between MMR and the development of autism in children with or without a sibling who has autism," writes King.

"Taken together, some dozen studies have now shown that the age of onset of ASD does not differ between vaccinated and unvaccinated children, the severity or course of ASD does not differ between vaccinated and unvaccinated children, and now the risk of ASD recurrence in families does not differ between vaccinated and unvaccinated children."